1. Field of the Invention
The present invention relates to an optical apparatus having a mirror.
2. Description of the Prior Art
For example, in the electron beam exposure system, a wafer is mounted on a stage which can move in both the vertical and horizontal directions in a plane for the irradiation of the desired regions by the electron beam.
In regard to this stage, it is essential to detect the position of stage with high accuracy in order to irradiate the electron beam accurately on the wafers. A laser interferometer is often used for accurately detecting the position of the stage.
The laser interferometer uses a laser generator, half mirror, etc. Thereby, the location of the stage can be detected by counting the interference fringes generated by the laser beam radiated from the laser generator and the laser beam which is radiated from the laser generator and then reflected at the stage. For the purpose of detecting the accurate location of the stage using such a laser interferometer, the stage is provided with a mirror which reflects the laser beam. The usual method of mounting this mirror to the stage will be explained below. FIG. 1a and FIG. 1b show the existing mirror mounting methods.
On a stage 1, consisting of the light weight metal such as aluminium alloy or magnesium alloy, the wafer 4 on which the electron beam is irradiated is mounted and a couple of mirrors 2a, 2b arranged in directions orthogonally crossing, in the case of FIG. 1a, are fixed on the stage 1 by the end portions 3a, 3b, 3c, 3d of the mirrors being screwed or by the bonding method using a bonding agent. On the other hand, an L-shaped mirror 2C can be used (FIG. 1b). This mirror is also fixed in the same way as in FIG. 1a at the end portions of the L-shaped mirror at 3e, 3g, and at the corner portion thereof 3f.
Stage 1 is usually composed of material such as aluminium alloy, as explained above, and the mirror, on the other hand, is composed of material such as quartz glass.
When comparing the linear expansion coefficients of the mirror and the stage, a large difference can be found because the quartz glass shows a value of 7.times.10.sup.-7 /.degree. C., while the aluminium shows a value of 23.9.times.10.sup.-6 /.degree. C.
If the temperature changes while the wafer is irradiated by the electron beam, the points of the mirror attached to the stage follow the expansion and contraction of the stage, resulting in deformation of the mirror as indicated by the broken line of FIGS. 1a and 1b. Thereby, the crossing angle of mirrors in the vertical and horizontal directions which should naturally be crossing orthogonally is deviated from the desired 90 degrees. When the electron beam is irradiated on the wafer and the pattern is formed, the pattern formed is deformed in the vertical and horizontal directions because the positioning of the electron beam is carried out with reference to the laser beam reflecting surface of this mirror.
Thus, it may be possible to employ such a structure that only the corner portion 3f of the L-shaped mirror is fixed and the end portions 3e and 3g are free, for example in the case of FIG. 1b, in order to prevent distortion of the mirror. However, fixing the mirror only at a single point would permit the free end portions of the L-shaped mirror to vibrate around such fixing point. As a result, accurate measurement is impossible.